1. Field of the Invention
The present invention relates to a semiconductor light-emitting device and a method of forming the same, and more particularly to a semiconductor laser device with a stripe-shaped mesa structure including a current injection center region and current non-injection side regions adjacent to facets and a method of forming the same.
2. Description of the Related Art
There has been known a semiconductor laser device with a stripe-shaped mesa structure including a current injection center region and current non-injection side regions adjacent to facets, wherein the current non-injection side regions are provided in order to avoid optical damages at the facets. The semiconductor device of this type will hereinafter be referred to as a facet non-injection type semiconductor laser.
FIG. 1 is a fragmentary schematic perspective view with a partially recessed view illustrative of a typical conventional internal structure of a facet non-injection type semiconductor laser. The semiconductor laser has an optical cavity which has a stripe-shaped ridge waveguide structure. The semiconductor laser is provided over an n-type semiconductor substrate 1.
An n-type cladding layer 2 overlies the n-type semiconductor substrate 1. An n-type etching stopper layer 3 overlies the n-type cladding layer 2. An n-type inner cladding layer 4 overlies the n-type etching stopper layer 3. An SCH-structure 5 including an active layer is provided over the n-type inner cladding layer 4. A p-type inner cladding layer 6 overlies the SCH-structure 5. A p-type etching stopper layer 7 overlies the p-type inner cladding layer 6. A stripe-shaped ridge optical waveguide of a p-type cladding; layer 8 is selectively provided on a stripe-shaped selected region of an upper surface of the p-type etching stopper layer 7. The stripe-shaped ridge optical waveguide extends in a longitudinal direction of the semiconductor laser.
A p-type cap layer 9 is selectively provided on a selected center region, except on both side regions adjacent to both facets, of the stripe-shaped ridge optical waveguide of the p-type cladding layer 8. A first current blocking layer 10 of n-type is provided over the both side regions of the stripe-shaped ridge optical waveguide, and over the p-type etching stopper layer 7 as well as in contact with side walls of the stripe-shaped ridge optical waveguide of the p-type cladding layer 8. The first current blocking layer 10 extends except over the p-type cap layer 9 selectively provided on the upper surface of the selected center region of the stripe-shaped ridge optical waveguide. A second current blocking layer 11 of n-type overlies the first current blocking layer 10.
A p-type contact layer 12 overlies the second current blocking layer 11 and the p-type cap layer 9. An n-electrode 13 is provided in contact width a bottom surface of the n-type semiconductor substrate 1. A p-electrode 14 is provided in contact with a top surface of the p-type contact layer 12. A current or carrier is injected through the p-type cap layer 9 and the selected center region of the stripe-shaped ridge optical waveguide 8 into the active layer of the SCH-structure 5. The first and second current blocking layers 10 and 11 prevent current injections directly into the both side regions of the stripe-shaped ridge optical waveguide 8 adjacent to the bot facets.
The selected center region of the stripe-shaped ridge optical waveguide 8 may be regarded as a current injection region. The side regions of the stripe-shaped ridge optical waveguide 8 covered by the first and second current blocking layers 10 and 11 may be regarded as current non-injection regions. The current is directly injected through the p-type cap layer 9 into the current injection region, and the injected current is then diffused into the current non-injection regions underlying laminations of the first and second current blocking layers 10 and 11.
Parts of the diffusion currents in in-plane directions through the current non-injection regions are supplied to the both facets. Namely, parts of the diffusion currents in parallel to the longitudinal direction reach the opposite facets. The stripe-shaped ridge optical waveguide 8 has a sheet resistance to currents in parallel to the longitudinal direction toward the opposite facets, wherein the longitudinal direction is parallel to a traveling direction of the stripe-shaped ridge optical waveguide 8. As this sheet resistance is high, then a density of the injection current to the facet is low. As the density of the injection current to the facet is low, then a Joule heat generated per a unit time is low. The generated heat is low, then a temperature increase at the facets is low. This contributes to delay a deterioration of the facets due to optical damages upon a high output operation.
Meanwhile, it has also been known that in order to obtain a high output of the semiconductor laser, the ridged waveguide is designed to increase, the thickness for obtaining a large spot size. In order to avoid the increase in resistance of the device upon the increase in thickness of the ridged waveguide, the ridged waveguide is also designed to increase a doping concentration for reducing a resistivity of the ridged waveguide. The increase in doping concentration of the ridged waveguide decreases a sheet resistance of the current non-injection regions of the ridged waveguide. The decreased sheet resistance of the current non-injection regions increases the density of the current through the to the current non-injection regions toward the facets, whereby the Joule heat generated per a unit time is increased and the temperature increase at the facets is high. This promotes the deterioration of the facets due to optical damages upon a high output operation.
It has also been proposed to counter-measure this problem, wherein the current non-injection regions are decreased in height, whilst the current injection region is increased in height. FIG. 2 is a fragmentary schematic perspective view with a partially recessed view illustrative of a conventionally modified internal structure of the facet non-injection type semiconductor laser. A structural difference of the semiconductor laser shown in FIG. 2 from that shown in FIG. 1 is only in that the ridged waveguide 8 is modified height, so that the current injection region is larger in height or thickness than the current non-injection regions.
The modified ridged waveguide 8 of FIG. 2 may be formed by selective etching to the cladding layer only on the current non-injection regions prior to shaping the ridged waveguide. FIGS. 3A through 3E are fragmentary schematic views of semiconductor laser devices in sequential steps involved in the conventional fabrication method for the semiconductor laser device of FIG. 2.
With reference to FIG. 3A, an n-type cladding layer 2 is formed over the n-type semiconductor substrate 1. An n-type etching stopper layer 3 is formed over the n-type cladding layer 2, An n-type inner cladding layer 4 is formed over the n-typo etching stopper layer 3. An SCH-layer 5 including an active layer is formed over the n-type inner cladding layer 4, A p-type inner cladding layer 6 is formed over the SCH-layer 5. A p-type etching stopper layer 7 is formed over the p-type inner cladding layer 6. A p-type cladding layer 8 is formed over the p-type etching stopper layer 7. A p-type cap layer 9 is formed over the p-type cladding layer 8.
A silicon dioxide film 15 is selectively formed on a selected center region of an upper surface of the p-type cap layer 9 except on both side regions separated from each other by the selected center region in the longitudinal direction. The silicon dioxide film 15 is used as a mask for carrying out a selective etching to the p-type cap layer 9 and an upper region of the p-type cladding layer 8. A depth of the etching is an intermediate level of the p-type cladding layer 8, so that the p-type etching stopper layer 7 is covered by the p-type cladding layer 8.
With reference to FIG. 3B, an additional silicon dioxide film is formed entirely which covers the etched surfaces on the side regions of the p-type cladding layer 8, and the etched side walls of the upper region of the p-type cladding layer 8 and the p-type cap layer 9 as well as on the original silicon dioxide film 15. The silicon dioxide film is then selectively etched by a lithography technique to form a stripe-shaped silicon dioxide mask 15 which extends along a longitudinal center line and has a constant width. The stripe-shaped silicon dioxide mask 15 thus includes a center region 15a and side regions 15b which are separated from each other by the center region in the longitudinal direction. The center region 15a extends over the upper surface of the p-type cap layer 9. The side regions 15b extends over the etched surfaces on the side regions of the p-type cladding layer 8. The center region 15a has a higher level than the side regions 15b. 
With reference to FIG. 3C, the p-type cap layer 9 is selectively etched by a wet etching process, so that the p-type cap layer 9 remains only under the center region 15a of the stripe-shaped silicon dioxide mask 15, and the top surface of the p-type cladding layer 8 is exposed, except under the remaining p-type cap layer 9 under the center region 15a of the stripe-shaped silicon dioxide mask 15. The wet etching process may be carried out by use of a first etchant which includes a mixture of a citric acid solution and a hydrogen peroxide at low mixing ratio. The remaining p-type cap layer 9 has a mesa stripe shape.
With reference to FIG. 3D, the p-type cladding layer 8 is selectively etched by a wet etching process, so that the p-type cladding layer 8 remains only under the center region 15a and the side regions 15b of the stripe-shaped silicon dioxide mask 15, and the top surface of the p-type etching stopper layer 7 is exposed, except under the remaining p-type cladding layer 8 under the stripe-shaped silicon dioxide mask 15. The wet etching process may be carried out by use of a second etchant which includes a mixture of a citric acid solution and a hydrogen peroxide at high mixing ratio. The remaining p-type cladding layer 8 also has a mesa stripe shape. The remaining p-type cap layer 9 receives a side etch by the second etchant. The mesa stripe shaped p-type cladding layer 8 forms a waveguide mesa structure.
With reference to FIG. 3E, the silicon dioxide film 15 is selectively removed so that the center region 15a remains over the remaining p-type cap layer 9.
As well illustrated in FIG. 3D, the mesa stripe shaped p-type cladding layer 8 comprises a center region 8a and side regions 8b separated from each other by the center region 8a. The center region 8a of the mesa stripe shaped p-type cladding layer 8 underlies the mesa-striped p-type cap layer 9 tinder the center region 15a of the stripe-shaped silicon dioxide mask 15. The side regions 8b of the mesa stripe shaped p-type cladding layer 8 underlie the side regions 15b of the stripe-shaped silicon dioxide mask 15. Not only, the center region 8a is larger in height than the side regions 8b, but also the center region 8a is wider in bottom width than the side regions 8b for the following reasons.
Before the second wet etching process, the p-type cladding layer 8 has different thicknesses as illustrated in FIG. 3C. The center region 8a of the p-type cladding layer 8 is thicker than the side regions 8b thereof, for which reason the center region 8a is larger in height than the side regions 8b. Further, the difference in height between the center region 8a and the side regions 8b results in that the side regions 8b are earlier removed to expose the top surface of the p-type etching stopper layer 7 on the center region, before the center region 8a is then removed to expose the top surface of the p-type etching stopper layer 7 on the side regions. Namely, on the side regions, the wet etching depth earlier reaches the top surface of the p-type etching stopper layer 7. After the wet etching depth has reached the top surface of the p-type etching stopper layer 7, then the side etching rate is increased. Therefore, the side regions 8b receive a larger side etch than the center region 8a. The side regions 8b become narrow as compared to the center region 8a. 
Accordingly, the center region 8a is wider in width than the side regions 8b. The mesa stripe shaped p-type cladding layer 8 serving as the ridged waveguide has discontinuations in height and width at boundaries between the center region 8a and the side regions 8b. The center region 8a corresponds to a current injection region of the ridged waveguide. The side regions 8b correspond to current non-injection regions of the ridged waveguide.
The discontinuation in width at boundaries between the current injection center region 8a and the current non-injection side regions 8b causes a coupling loss at the boundaries in a horizontal waveguide mode. This coupling loss causes some disadvantages in high output operation of the laser device, for example, the increase of the oscillation threshold current and the drop of the slope efficiency.
Further, the discontinuation in height or height at boundaries between the current injection center region 8a and the current non-injection side regions 8b causes another coupling loss at the boundaries in a vertical waveguide mode. Notwithstanding, the reduction in height of the current non-injection side regions 8b reduces a Joule heat generation at the facets. In view of suppressing any excess coupling loss at the boundaries in a vertical waveguide mode, however, it is necessary to avoid any excess reduction in height of the current non-injection side regions 8b. 
Furthermore, the discontinuations in both thickness or height and width at boundaries between the current injection center region 8a and the current non-injection side regions 8b cause that a difference in effective refractive index between inside and outside of the mesa structure of the waveguide 8 is different between on the current injection center region 8a and on the current non-injection side regions 8b. This difference causes the coupling loss at the boundaries in the horizontal waveguide mode. In view of suppressing any excess coupling loss at the boundaries in the horizontal waveguide mode, however, it is necessary to avoid any excess reduction in height of the current non-injection side regions 8b. 
For the above-described reasons, there is a limitation to reduce the height of the current non-injection side regions 8b, although the reduction in the height of the current non-injection side regions 8b would be effective to suppress the Joule heat generation at the facets.
As long as the laser device has the above-described conventional waveguide structure, the emphasis of suppressing the Joule heat generation at the facets causes the coupling losses in the horizontal and vertical waveguide modes, and the disadvantages in high output operation of the laser device, for example, the increase of the oscillation threshold current and the drop of the slope efficiency.
In the above circumstances, the development of a novel semiconductor light emitting device free from the above problems is desirable.
Accordingly, it is an, object of the present invention to provide a novel semiconductor light emitting device free from the above problems.
It is a further object of the present invention to provide a novel semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the semiconductor light emitting device is free from the above problems.
It is a still further object of the present invention to provide a novel semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of highly suppressing Joule beat generation at facets.
It is yet a further object of the present invention to provide a novel semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of avoiding a substantial reduction in coupling loss in horizontal waveguide mode.
It is further more object of the present invention to provide a novel semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of avoiding a substantial reduction in another coupling loss in vertical waveguide mode.
It is more over object of the present invention to provide a novel semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of suppressing the oscillation threshold current.
It is still more object of the present invention to provide a novel semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of suppressing the drop of the slope efficiency.
It is an additional object of the present invention to provide a novel method of forming a semiconductor light emitting device free from the above problems.
It is further additional object of the present invention to provide a novel method of forming a semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the semiconductor light emitting device is free from the above problems.
It is still further additional object of the present invention to provide a novel method of forming a semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of highly suppressing Joule beat generation at facets.
It is yet further additional object of the present invention to provide a novel method of forming a semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of avoiding a substantial reduction in coupling loss in horizontal waveguide mode.
It is further more additional object of the present invention to provide a novel method of forming a semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of avoiding a substantial reduction in another coupling loss in vertical waveguide mode.
It is more over additional object of the present invention to provide a novel method of forming a semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of suppressing the oscillation threshold current.
It is still more additional object of the present invention to provide a novel method of forming a semiconductor light emitting device having a ridge waveguide structure which comprises a current injection center region and current non-injection side regions which are smaller in height than the current injection center region, wherein the ridge waveguide structure is capable of suppressing the drop of the slope efficiency.
The present invention provides a ridge waveguide structure of a cladding layer in a semiconductor light emitting device. The ridge waveguide structure comprises: at least a current injection region; and current non-injection regions adjacent to facets and separating the current injection region from the facets, wherein the current non-injection regions are smaller in height than the at least current injection region, and wherein the at least current injection region and the current non-injection regions have a uniform width at least at a lower level than an intermediate level of the ridge waveguide structure.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.